1. Technical Field
The present invention relates generally to photoreceptors designed to operate at low light levels and more particularly to circuits for increasing the reliability of photoreceiver information acquired under conditions in which a signal-to-noise ratio is relatively small.
2. Related Art
An accurate determination of the path of a device across a surface is important in a variety of applications. For example, if a faithful representation of an image of a scanned original is to be acquired, there must be accurate information as to the travel of the scanning device along the original. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array.
U.S. Pat. No. 5,149,980 to Ertel et al., which is assigned to the assignee of the present invention, describes use of a cross-correlation function to determine the relative movement between an original and an array of photoelements in a given direction. The patent notes that the one-dimensional approach can be extended to determine the vector of two-dimensional relative movement between the original and the array, so as to track translation, rotation and scaling in a two-dimensional plane.
The patent to Ertel et al. describes use of an optical sensor array to collect a "signature" of an original. The signature may be provided by illuminating and imaging the surface texture or other optical characteristics of the original. The light intensity will vary on a pixel-by-pixel basis with variations in surface texture. By cross-correlating images of the surface of the original, relative movement between the array and the original can be ascertained.
A critical element of the design of a system such as the one described by Ertel et al. is circuitry to maintain the signal-to-noise ratio of each photoelement sufficiently high to reliably determine the signature of the original. If the signal is the difference in reflectivity from pixel to pixel as a result of slight variations in paper texture of a white paper, the variation in reflectivity may only be approximately six percent. The overall resolution goals translate into a relatively low signal-to-noise ratio for each photoelement, with the desired signal being the small change in reflectivity of the medium of interest and the dominant noise term being shot noise of the photodiode as a result of the fixed portion of the reflectivity. The reference to Ertel et al. uses averaging to increase the signal to noise ratio to obtain useful information.
U.S. Pat. No. 5,769,384, filed on Jan. 25, 1996, to Baumgartner et al., which is assigned to the assignee of the present invention, also describes the use of circuitry for an optical sensor array to collect a "signature" of an original. Applicants hereby incorporate U.S. Pat. No. 5,769,384 by reference in its entirety. A critical element of the Baumgartner et al. circuitry is a servo circuit in combination with a photoelement for biasing a base node via an emitter node of a phototransistor. The phototransistor base node is thereby maintained at a substantially fixed voltage, so that photocurrent generated by reception of light energy is directed to a pixel storage or signal integration capacitor, rather than being used to charge and discharge parasitic capacitance of the phototransistor. Thus, the servo circuit of the photoelement maintains the bias point on the phototransistor base to efficiently deliver the light-induced current to the appropriate integration capacitor. Additionally, that circuit layout design achieves a high photoreception area.
However, although meritorious to an extent, the servo circuit of Baumgartner et al. includes several disadvantages. First, a direct result of the large base area of Baumgartner et al. is a large associated collector-base capacitance. That capacitance causes the servo circuit to respond very slowly to perturbations in the voltage of the base node. Furthermore, servo circuit response time is also a function of the ambient light falling on the photoelement. Therefore, the ability of the servo circuit to respond to dynamic variations in the incident light on the photoelement base is a function of the average or DC level of the incident light. Thus, if the incident light level is low, as it is in some image acquisition applications, then the servo circuit output bandwidth is also unacceptably low.
Furthermore, in Baumgartner et al., the optical sensor array includes a number of cells, wherein each cell includes one or more photoelements having associated integration capacitors. In the Baumgartner et al. system, integration capacitors are read in a distributed time period, by means of a read control line which signals the transfer of integration capacitor charge to a transfer amplifier. Thus, integration time for each integration capacitor depends on the length of the read time period. During image acquisition, however, each cell, or row of cells integrates at a different time although within the read time period, which causes image smearing across the array of photoelements.
What is needed is circuitry which permits reliable use of a photoelement signal in applications in which small light-intensity differentials are employed as application information.
Other objects, features and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.